Transient replicative properties of the expression vectors. CHO cells were electroporated with 1, 2 and 5 μg of p3.7LDL (lanes 2,3,4) and the empty self-replicating vector TK3.7 (lanes 6, 7 8). Low-molecular weight DNA was digested with DpnI and the linearising enzyme Bpu1102I. Fragments were analysed by Southern blotting. The corresponding marker DNAs (200 pg each) were digested with Bpu1102I (lanes 1 and 5). Hybridisation probe was a 1 kb BPV-1 origin fragment. Exposure 12 hrs.